Prayers have been said at the Hagia Sophia for the first time in 86 years.
The historic Hagia Sophia is located in Istanbul. It was built as a church, then became a mosque and finally a museum.
The site is 1500 years old and became a Unesco World Heritage museum in 1934.
However, the site has been changed after Turkish authorities ruled it could be converted into a mosque.
Clerics in white robes sang from the Koran. Mosaics and images of Jesus and Mary were draped with curtains.
The Hagia Sophia was built as an Orthodox Christian cathedral and first converted into a mosque after the Ottoman conquest nine centuries later.
The decision to turn it back into a mosque has been contentious. In neighbouring Greece, Prime Minister Kyriakos Mitsotakis passed on a message of sorrow to millions of Greek Orthodox Christians.